Semiconductor laser devices which emit red light are of great commercial importance, and are used in areas such as data storage. The active region of such a laser device is commonly made using crystalline semiconductors of the (Al,Ga,In)P alloy system. The laser is usually grown on a GaAs substrate, and other GaAs layers may be used in the laser structure.
One common laser structure is the buried ridge laser. Buried ridge lasers are used for optical disc data storage applications which require stable single mode operation. It is known to make a buried ridge laser by means of a procedure involving three semiconductor deposition stages. First, multilayers of (Ga,In)P and (Al,Ga,In)P are deposited on an n-type GaAs substrate to form an n-type cladding region, an active region and a p-type cladding region of the laser. Then, a ridge is etched into the p-type cladding region. Then, n-type GaAs is deposited over the ridge and selectively etched away over the top of the ridge to expose a (Ga,In)P layer at the top of the ridge. Finally, p-type GaAs is deposited over the whole of the structure to bury the ridge, and then metal contact layers are deposited.
A buried ridge laser is disclosed in Japanese patent application No. JP-03 156592 which requires only two semiconductor deposition stages. Multilayers of (Ga,In)P, (Al,Ga,In)P and GaAs are deposited on a GaAs substrate to form the active and cladding regions. Then, after the ridge has been formed by etching, n-type GaAs is deposited over the whole of the structure and etched away from the top of the ridge. Metal contact layers are then deposited. The reduction in the number of semiconductor deposition stages is an advantage for mass production.
The ridge may be etched using either a chemical solution or a plasma. Chemical etching is, however, unsatisfactory where the deposited layers are misoriented. It is advantageous to deposit each layer such that it is slightly misoriented from the nominal (100) crystal direction of the underlying layer, because undesirable "ordering" of the crystal can be suppressed by using such misorientation. Misorientation towards the (110) crystal direction of 4.degree. to 15.degree. and even higher is possible.
A chemical solution will etch different crystal planes at different rates. Ridges formed in misoriented layers by chemical etching are found to be asymmetric, and this degrades the properties of the laser device. In particular, it is found that the peak of the far-field emission pattern is shifted away from the direction normal to the laser facet if the ridge is asymmetric, which creates difficulties in coupling to the output of the laser. In addition, the chemical etching procedure has a number of steps and can be difficult to control in a production process.
Plasma etching does not usually discriminate between crystal planes, so this process can be used to form a ridge in misoriented layers. Other advantages of plasma etching are that it may be accurately controlled, and may be readily incorporated into a production process. Not all plasma etching processes will be appropriate, however, because some cause damage to the remaining semiconductor. in addition, some processes may selectively etch the various layers in the laser device at different rates and this may cause the etched ridge to be "stepped" at the interface between the (Al,Ga,In)P and GaAs layers, rather than having a smooth continuous profile that is required.
"Journal of Vacuum Science and Technology", Vol. B1 pages 1053-1055 (1983), discloses the use of SiCl.sub.4 plasmas and SiCl.sub.4 /Ar plasmas generated in a conventional rf-powered parallel-plate "Reactive Ion Etching" (RIE) system at total pressures of between 1 and 25 mTorr to etch both GaAs and InP. It also discloses that the addition of Ar in Ar:SiCl.sub.4 flow ratios of 4:1 , 2:1 and 1:4 eliminated trenching and redeposition observed in InP and resulted in more vertical walls as compared to the use of SiCl.sub.4 alone. However, this document discloses only the etching of individual layers and does not examine the profile obtained by etching a sample containing layers of both GaAs and (Al,Ga,In)P.
EP-A-0 547 694 teaches that the above-mentioned SiCl.sub.4 /Ar plasma etching technique is unsuitable for use on GaAs/(Al,Ga,In)P laser structures because it results in rough surfaces which are undesirable for optoelectronic devices. Such document discloses a onestep plasma etching process for forming the ridge of GaAs/(Al,Ga,In)P lasers using a mixture of SiCl.sub.4, Ar and CH.sub.4 gases in a rf-powered parallel-plate geometry, wherein the sample being etched is heated to a temperature of between 100.degree. and 150.degree. C. Such document also states that it is essential to add CH.sub.4 to the SiCl.sub.4 /Ar gas mixture if the smooth surfaces required for optoelectronic devices are to be obtained. In a practical etching process, however, the use of CH.sub.4 gas is undesirable because of the possibility of polymer formation on the etched surface and/or hydrogen diffusion into the semiconductor.
U.S. Pat. No. 5338394 discloses a method for etching indium-based III-V compound semiconductors in which electron cyclotron resonance (ECR) etching is effected in the presence of SiCl.sub.4 and H.sub.2 or CH.sub.4 in an ECR chamber at any pressure suitable for ECR, but with an expressed preference for use of a pressure between 1 mTorr and 5 mTorr.
EP-A-0607662 discloses a method of selectively etching GaAs over AlGaAs using a mixture of SiCl.sub.4, CF.sub.4, O.sub.2 and an inert gas such as He so as to avoid using a chlorofluorocarbon etchant in a standard plasma etching apparatus, but without any specific disclosure of the pressure employed apart from a reference to etching at 5 mTorr with no He.